The present invention relates generally to structured packings, and more particularly, to structured packing elements and applications to cryogenic distillation.
Structured packings have found widespread use in a variety of distillations including those involved in the separation of air into its component parts. Distillations are conducted within distillation columns filled with mass transfer elements to bring ascending vapor phases into intimate contact with descending liquid phases of mixtures to be separated. As the ascending phase rises and contacts the descending liquid phase, it becomes evermore enriched in the more volatile components of the mixture to be separated. At the same time, the descending liquid phase becomes ever more concentrated in the less volatile components of the mixture to be separated. In such fashion, systems of distillation columns can be used to separate various mixture components. For instance, in case of air separation, nitrogen is often separated from oxygen in a double distillation column unit, in which the descending liquid phase becomes ever more concentrated in oxygen and the ascending gaseous phase becomes ever more concentrated in nitrogen. Argon is then separated from oxygen in an argon column that is attached to a lower pressure column of such a double distillation column unit.
Structured packings are widely used as mass transfer elements within distillation columns due to their low pressure drop characteristics. Structured packings generally include a series of structured packing elements that are made up of a number of corrugated sheets of material, in which the sheets are placed in a side by side relationship with the corrugations of adjacent sheets criss-crossing one another. In use, the liquid phase of the mixture to be separated is distributed to the top of the packing and spreads out throughout the packing as a descending film. The vapor phase of such a mixture rises through the corrugations contacting the liquid film as it descends. The use of structured packings in distillation columns for air separation is well-known in the art, and a more detailed discussion can be found, for example, in U.S. Pat. No. 6,280,819, entitled xe2x80x9cStructured Packingxe2x80x9d.
The performance of a structured packing is characterized primarily by its separation efficiency, or the height of packing equal to a theoretical plate (HETP), its operational pressure drop, and its capacity. Packings with high capacity and high separation efficiency (low HETP) are desirable because they lead to reduced equipment sizes and lower capital costs in commercial plants. Packings with low operational pressure drops are desirable because they lead to reduced power requirements and thus lower operating costs. Since the performance characteristics of structured packings have such a great influence on the economics of distillation-based commercial processes, there are many examples of packings that have been designed in an attempt to achieve an optimal balance between separation efficiency, pressure drop and capacity for a given process.
The present invention provides generally a structured packing element comprising a plurality of perforated corrugated sheets and a planar member positioned adjacent to each of the corrugated sheets. The planar member has a top and bottom portion each having an open area percent that is less than that of a middle portion. Furthermore, the outermost edges of the top and bottom portions are at least proximally aligned with respect to the top and bottom edges respectively of the perforated corrugated sheets. In one embodiment, the open area percents of the top and bottom portions are each less than about 20%, the open area percent of the middle portion is at least about 50% and less than 100%, and the heights of the top and bottom portions are each at least about two times the corrugation width of the corrugations in the corrugated sheets, but not greater than about one-third of the height of the corrugated sheets.
Another aspect of the present invention provides for a structured packing comprising a plurality of these structured packing elements, in which the open area percents of the top and bottom portions of the planar member are each less than the open area percent of the middle portion, and the outermost edges of the top and bottom portions are at least proximally aligned with the top and bottom edges of the corrugated sheets. Structured packings can be fabricated from the packing elements of this invention with reduced costs and enhanced quality, and such packings further provide improved capacity compared to conventional packings. These structured packings can be used in distillation columns for a variety of applications, including cryogenic distillations such as air separation.
Thus, another aspect of the present invention provides for a method of cryogenic separation of a fluid mixture in a distillation column, in which at least one section of the distillation column contains a structured packing that comprises structured packing elements having a planar member adjacent to each of the corrugated sheets, where the planar member has a middle portion having an open area percent that is more than those of the top and bottom portions of the planar member, and the planar member has outermost edges that are aligned with the top and bottom edges respectively of the corrugated sheets.